In an electrographic image forming device in a conventional printer device, facsimile machine, photocopy machine and the like, an electrostatic latent image is formed on a surface of a photosensitive drum, which is an image carrier body, by illuminating light based on inputted print data using an exposure apparatus, such as a light emitting diode (LED) head, after uniformly charging the surface by a charge member, such as a charge roller.
Then, by electrostatically attaching a developer to the electrostatic latent image formed on the surface of the photosensitive drum by a developer supply member, such as a development roller, a developer image is inversely developed. The developer image formed on the surface of the photosensitive drum is transferred to a recording medium carried at a predetermined timing by a transfer member, such as a transfer roller. The recording medium, to which the developer image has been transferred, is carried to a fuser thereafter, and the developer image is fixed to the recording medium by applying heat or pressure. The recording medium, to which the developer image has been fixed, is ejected outside the image forming apparatus to complete the series of the image forming processes.
In the image forming apparatus having such a configuration, the fuser generally includes a fusion roller having a heat body as a heat source, and a pressure application roller positioned to apply pressure to the fusion roller. The fusion roller and the pressure application roller are maintained in a state to press each other, and heat and pressure are applied when the recording medium passes a nip part formed by the rotating fusion roller and pressure application roller. Herein, the nip part is defined as an area where the two rollers contact and where a sheet passing through the rollers is pressed.
Japanese Laid-Open Patent Application Publication No. 2002-132086, for example, provides an image forming device that includes two print modes: a high temperature and high speed (HTHS) print mode that sets a high temperature for the above-discussed fuser and rotates the fusion roller at high speed; and a low temperature and low speed (LTLS) print mode that sets a lower temperature for the fuser and rotates the fusion roller at a slower speed than the HTHS print mode. The image forming device performs the printing by switching between the HTHS print mode and the LTLS print mode in response to the number of sheets to be printed.
However, in the image forming device of the above related art, the printing is performed in the LTLS print mode, in which the rotation speed of the fusion roller is slower, when the printing number of sheets is less than a predetermined number even if the temperature of the fuser is sufficiently high. Therefore, there is a problem that a long time is required to complete the printing for such a printing mode.
In view of such a problem, the present invention has an object to provide an image forming device that reduces the time required to complete printing by performing the printing in an optimum print mode selected based on a result of detecting the surface temperature of the pressure application roller.